Coated femoral stem prosthesis

ABSTRACT

A femoral hip component for use in hip arthroplasty has a stem portion for contact with bone cement. The stem portion is formed from a biocompatible material having a surface finish of less than about four microinches. This surface is coated with a uniform layer of material selected from the group consisting of diamond-like carbon, chromium carbide, titanium nitride, titanium carbo-nitride, chromium and zirconium and a combination thereof.

This is a continuation of application Ser. No. 08/503,572, filed on Jul.18, 1995, now U.S. Pat. No. 5,593,452, which is a continuation ofapplication Ser. No. 08/189,629 filed on Feb. 1, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hip joint prosthesis and moreparticularly to a femoral component of such a prothesis which is coatedwith a material which reduces the bonding of the prosthesis to bonecement.

2. Description of the Prior Art

Many methods and devices have been developed to improve the fixation ofhip joint protheses including the femoral component thereof in the bodyso that the device implanted therein becomes as permanent as possible.Many orthopedic implants use a cement to anchor the stem portion of afemoral component in the femur. For example, United Kingdom PatentSpecification No. 1,409,054 in the names of Robin S. M. Ling and Alan J.C. Lee discloses a hip joint prosthesis having a double-tapered stemwhich, among other advantages, enhances extrusion of cement caused bypenetration of the stem during fixation. U.S. Pat. No. 3,793,650discloses an intramedullary stem for a prosthetic bone joint device ofthis type also having a base with spring members intended to centralizethe position of the stem in the canal or bore of the bone in order toinsure a relatively uniform or, at least minimum thickness of cementbetween the wall of the bone and the stem. It has been found desirableto have a uniform mantle of at least two millimeters (2 mm) of cementbetween the stem and the bone.

The prior art has shown centralizers as a means for insuring that therewill be at least a certain minimum thickness of cement between the stemof the prosthesis and the interior wall of the canal formed in the femurbone for receiving such stem, the likelihood of the stem protrudingthrough the cement and contacting the interior of the femur bone itselfis minimized. Thus, in those types of implants using cement, it isimportant to insure that the stem is completely encapsulated by thecement and does not protrude through to contact the bone.

One type of bone cement utilized to retain the stem of a femoral hipjoint prosthesis in the canal of a bone comprises a mixture ofpolymethylmethacrylate (hereinafter PMMA) polymer and methylmethacrylate monomer and optionally including a styrene co-polymer ofPMMA. This and other types of cement utilized for such purpose may bepackaged in two separate components which are mixed into a paste whichis placed in the canal of the femur immediately prior to insertion ofthe stem of the prosthesis. Such paste then sets to a relatively rigidmaterial providing excellent adherence to the interior wall of the bone.

In both the cemented and non-cemented types of devices used heretofore,problems have arisen, particularly after a number of years ofimplantation. It has been found that the cement utilized to retain thestem of the device in the canal of the femur bone is subject to aphenomenon known as creep. Thus, while the bone cement appears to berigid when set, it is subject to minute amounts of movement over time.The amount of creep encountered with such cement following implantationis exaggerated by virtue of the fact that the body temperature controlsthe temperature of the implanted cement and prosthesis. Thus, PMMA andother types of bone cement at body temperature are subject to a greaterdegree of creep than bone cement maintained at room temperature of, say,72° F. This may be readily observed by mounting a bar of PMMA so thatits ends are supported and applying a fixed load at the center of thebar. Tests have shown that a bar so supported and subjected to a load of5 pounds for eight hours at 98.6° F. will deflect to an extent 3.5 timesgreater than an identical bar supported and loaded in an identicalmanner for eight hours at 72° F.

Over a period of time, the phenomenon of creep may result in disruptionof the micro-interlocking of the cement-implant interface. This mayallow the prosthesis to loosen and cause unwanted movement. In addition,the femoral component could subside as the cement deformed over time.

In the early 1970's a polished femoral hip component was designed whichwas intended to subside as the bone cement deformed over time. This stemis shown in the afore-mentioned U.K. Patent 1,409,054 and sold under thetrade name Exeter® hip by Howmedica Inc.. With this design, theprosthesis is wedge-shaped and automatically relocks itself within thebone cement as subsidence occurs due to bone cement creep.

As discussed in U.S. Pat. No. 5,171,275, it is well known that polishingthe Exeter® hip stem allows for less adhesion between the bone cementand the prosthesis to permit subsidence. It has now been unexpectedlyfound that coating a polished prosthesis with the coatings of thepresent invention further reduces the bond between the prosthesis andthe bone cement.

Coatings such as diamond-like coatings are known and may be applied tometal by processes described in U.S. Pat. Nos. 4,382,100, 4,394,400 and4,645,977, the teachings of which are incorporated herein by references.The diamond-like films produced by these methods on a metal substrateare known to reduce friction (see U.S. Pat. No. 4,525,417). The coatingof metallic orthopedic implants has been broadly taught in EPOpublication 0 302 717 A1 and Japanese Patent Application 59-82851 (1984)but these publications did not address the advantages of using such acoating on a polished hip stem of the type disclosed in U.K. 1,409,054designed to subside as bone cement creeps.

SUMMARY OF THE INVENTION

The present invention provides for a femoral hip joint prosthesis havinga design which allows for subsidence of the stem within the cementmantle by reducing the bond between the prosthesis and the bone cement.

Accordingly, it is an object of the present invention to provide a newand novel femoral hip joint prosthesis which is specifically designed tomarkedly reduce the bond between the prosthesis and the bone cement bycoating the prosthesis with an adhesive reducing composition.

It is a further object of the present invention to provide a femoral hipjoint prosthesis which will not loosen but rather will self-tighten,even though the cement mantle creeps or expands fractionally over aperiod of time.

It is yet another object of the present invention to provide a femoralhip joint prosthesis in which the stem subsides within the cement as thecement creeps and thus is permitted to remain at all times in snuginterfacial contact therewith, imparting in the stem area the reliablecompressive forces against the cement which is micro-interlocked withthe bony surfaces.

Finally, it is an object of the present invention to provide a femoralhip joint prosthesis in combination with a cement mantle implanted inthe canal of a femur herein said cement mantle encapsulates the stem ofsuch prosthesis in an interfacial relationship which permits subsidenceof the stem within the cement mantle without disrupting the interfacialadherence between the cement mantle and the bone.

The femoral hip joint prosthesis of the present invention is collarless,has a double tapered stem formed in the preferred embodiment, and hasthe surface of the stem highly polished to provide an extremely smoothsurface. The stem is coated with a coating, for example, a layer ofdiamond-like carbon. It has been discovered unexpectedly that thesecoatings like Diamond-Like-Carbon (DLC) produce a highly abrasionresistant surface that has minimal adhesion to bone cement. This was anunexpected result, as many epoxy cements (other than PMMA) adhere quitereadily to abrasion resistant coatings like DLC. Thus the application ofDLC to the surface of a polished hip stem should result in increasedfatigue strength, increased abrasion resistance, reduced adherence, andless bone cement debris. The lower end of the stem may be positioned ina hollow centralizer which serves to stabilize it and insures that anadequate thickness of cement encapsulates the stem. Such design permitsthe stem portion of the prosthesis to move fractionally within thecement mantle without disrupting the cement-bone interface and toself-tighten as the male component, namely, the distal tip of the stemengages further in the hollow centralizer.

The prior art's highly polished, tapered shape results in a low adhesionbetween the prosthesis and the PMMA bone cement. As a result of thisreduced adhesion, bone cement debris generated by motion at themetal-cement interface are reduced. When adhesion is reduced evenfurther, then wear debris are minimized. In addition, resistance tosubsidence is reduced.

Although prior art prostheses such as the tapered collarless bone jointdevices disclosed in the previously referenced United Kingdom PatentSpecification No. 1,409,054 and U.S. Pat. No. 3,793,650 have been usedwith polished surface, they have never utilized an outer layer of, forexample, diamond-like carbon to further reduce the bone between theprosthesis and the bone cement. The prosthesis of the present inventionprovides superior results over the prior art in that, as well asallowing enhanced subsidence within the cement mantle, it exhibits goodcorrosion resistance when implanted in the body. Furthermore, thecoatings taught herein have a surface roughness which mimics that of theunderlying polished surface.

These and other objects and advantages of the present invention willbecome apparent from the following description of the preferredembodiment, which discloses several embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred femoral hip joint prosthesis has a stem which isconvergently tapered toward a distal end and extending along a firstaxis of symmetry A to an area of juncture with a neck portion lying on asecond axis of symmetry B. The stem, in the area to be in contact withthe bone cement, is polished, preferably to an average roughness of lessthan about 4 microinches and then is coated with a layer of adhesionreducing coating, for example, diamond-like carbon coating at least 100Å thick and preferably about 1 to 3 inches microns. This coating may beapplied by any well known method such as that disclosed in U.S. Pat. No.4,382,100 and is preferably uniform and non-porous.

For example, a method of applying a carbonaceous material to aprosthesis surface may consist of placing the surface in an enclosurecontaining a gas at less than atmospheric pressure, the gas consistingsubstantially of carbon and hydrogen; and simultaneously generating aplasma in the gas in said enclosure and applying to the surface throughcapacitive means an electrical potential which changes sign at timeintervals of between 5×10⁻⁹ and 10⁻⁶ seconds.

The surfaces of the prostheses may be of conducting or semiconductingmaterial, when the capacitive means comprises a separate capacitor, orthe surfaces may be of an insulating material, when the bodies ofmaterial may themselves comprise the capacitive means.

The gas will normally be a hydrocarbon compound with the optionaladdition of a small proportion of another gas if a "doped" carbonaceouslayer is required on the surface.

The plasma may be generated in a two electrode system by a source ofradio frequency or may be generated in a three electrode system byseparate means, for example an additional radio frequency source or ahot cathode or a cold cathode glow discharge arrangement.

In a two-electrode system, the plasma is generated by connecting theprosthesis surface through capacitive means to one terminal of a sourceof electromagnetic radiation at a frequency of between 0.5 and 100megahertz, and connecting the other terminal of the source to anelectrode spaced from the surface.

The preferred prosthesis further includes a neck portion which is afrustoconically shaped Morse taper neck to which may be attached aspherically shaped Morse taper head. No collar is provided in thefemoral hip prosthesis, but rather the portion of the prosthesis joiningthe stem to the neck follows a smooth arcuate contour in the area of theincluded angle between the respective axes of symmetry A and B. Theportion of the femoral hip prosthesis opposite the smooth arcuateportion, namely, that portion on the outside of the angle between thetwo axes of symmetry A and B, has an enlarged shoulder.

Preferably, an aperture may be provided in the area of the neck andshoulder to assist in removing the prosthesis in the event revision isrequired at some future time.

The stem is tapered in both directions and preferably has roundedcorners. As pointed out in United Kingdom Patent Specification1,409,054, such double tapering enhances the extrusion of cement causedby penetration of the stem thereinto during fixation.

The femoral hip joint prosthesis of the present invention may be formedof high strength forged Co-Cr-Mo alloy (ASTM designation F-799) and hasits surface polished to a high degree (also known as a buff finish) toprovide for a smoothness having a target surface roughness preferably ofabout four (4) microinches. Of course, stainless steel or otherbiocompatible materials, even composites, can be used to make the stembase material. The coated polished Vitallium® Co-Cr-Mo alloy orstainless steel stem is superior to metal alone because the coatingbetter resists pitting and crevice corrosion of the metal in the bodyenvironment as well as reducing the bone strength between the bonecement and the prosthesis.

It is the combination of the Co-CR-Mo alloy having its surface andpolished and coated with, for example, diamond-like carbon coupled withthe tapered stem and collarless design which permits the tapered stemand collarless design which permits the femoral hip prosthesis of thepresent invention to function in the manner intended without looseningand without causing pain or other adverse mechanical effects in thepatient even though there is subsidence of the prosthesis over a periodof time. Thus, the present design permits the polished and coated stemto subside within the cement mantle. The taper of the stem permits it toself-tighten upon the slight movement which occurs during the subsidenceand engage in the hollow centralizer and yet to do so without pullingthe cement mantle and thus avoid disrupting the micro-interlocking atthe cement-bone interface. Such design causes the stem to impartprimarily compressive forces against the cement mantel, thustransmitting the load to the femur. Transmitting the load in this mannerforces the cement mantle continuously snugly and firmly against theinterior of the femur to assist in maintaining the integrity of themicro-interlocking at the cement-bone interface.

EXAMPLE

In order to test the effect of diamond-like coatings on a cobalt chromestem, diamond-like carbon coatings were applied to highly polished (lessthan about 4 microinches) Vitallium® (CoCr) discs by Diamonex Inc., ofAllentown, Pa. as a commercial service, using diamond-like carbondeposition technology similar to that discussed above. These coatingswere greater than 100 Å preferably and up to several microns forimproved wear purposes. The maximum thickness of the coatings is notmaterial to the reduction of bond strength between the prosthesis andthe bone cement as long as the surface finish is maintained. Uncoated,highly polished Vitallium discs were used as controls. All the discswere ultrasonically cleaned in methanol and dried. Size 0.7134 cmdiameter aluminum "pull studs" having a flat bottom surface were abradedwith 180 grit and sand paper, washed with methanol, and dried.

The Howmedica Simplex P® bone cement was mixed per manufacturersinstructions using a Howmedica Simplex® enhancement mixer. Sufficientmixed cement was placed on the flat portion of the pull stud to permitcomplete coverage of the pull studs face when pressed against the testspecimens. The cement coated pull stud was pressed against the surfaceof the polished test specimens and held in place with a metal clip untilthe cement cured completely, approximately two hours.

The pull stud with the attached test specimen was placed in a SebastianFive-A materials tester and fixed in the tester's holder as permanufacturers instructions. The pull rate was set for 28.57 Kg/min.After completion of the test cycle, the force required to pull off thetest specimen was recorded. The testing equipment output was convertedto adhesive strength by dividing the displayed pull off force (inKilograms) by the attachment area of the pull stud (in squarecentimeters) to yield a value in units of Kg/cm².

The following table shows the adhesive strength of both diamond-likecarbon coated polished Vitallium® cobalt chrome and untreated polishedVitallium® control discs. In addition, other coatings such as Chromiumcarbide, Chromium nitride, Titanium nitride, Titanium carbon nitride,Chrome and Zirconium were used to coat a polished Vitallium® hip stem.These coatings were also greater than 100 Å and up to about 3 microns.

                  TABLE                                                           ______________________________________                                        TENSILE ADHESION VALUES OF PMMA BONE CEMENT                                   TO VARIOUS COATED VITALLIUM SUBSTRA                                           All values are in Kg/cm.sup.2                                                 ______________________________________                                        DLC Coated Vitallium                                                          ______________________________________                                        Sample   1         18.7                                                                2         7.2                                                                 3         5.7                                                                 4         20.0                                                                5         15.0                                                                6         13.6                                                                7         12.4                                                       Average of 7 samples                                                                         13.23                                                          ______________________________________                                                       Average Values                                                                            Commercially                                       Coated Vitallium (non-DLC)                                                                   Kg/cm.sup.2 Available From:                                    ______________________________________                                        Chromium Carbide*                                                                            2.8         *Richter Precision,                                                           East Petersburg, PA                                Chromium Nitride**                                                                           0           **Balzers,                                                                    Mt. Clement, MI                                    Titanium Nitride***                                                                          38.1        ***Richter Precision                                                          and Balzers                                        Titanium Carbo-Nitride**                                                                     18.1                                                           Amorphous Metallic                                                                           49.7        ****Electrolyzing                                  Chromium****               Inc., Providence, RI                               Zirconium, Ion 54.2        *****Implant Sciences                              Implanted*****             Inc., Wakefield, MA                                ______________________________________                                        Uncoated Vitallium Controls                                                   ______________________________________                                        Sample   1         60.0                                                                2         56.0                                                                3         40.4                                                                4         101.8                                                               5         87.8                                                                6         99.1                                                                7         96.8                                                                8         82.1                                                                9         95.6                                                                10        117.6                                                               11        117.3                                                               12        100.1                                                               13        99.4                                                       Average of 13 Samples                                                                        88.8                                                           ______________________________________                                    

The interface adhesive strength between the Vitallium with thediamond-like coatings and the Simplex P® bone cement was unexpectedlyalmost four times less than the interface strength between Simplex bonecement and uncoated Vitallium® control samples.

The table also shows the adhesive strength of the other coatings (allapplied commercially at the locations listed) at thicknesses of greaterthan 100 Å and preferably 1 to 3 microns and more preferably 1 micron,which also reduce the bond strength of the bone cement to the Vitallium.All were less than the Vitallium control adhesive strength of Table I.

Again, the bond or adhesive strength between these coatings on theVitallium and the bone cement is less than that of the control sample.While highly polished Vitallium was used in the above tests, otherpolished metals such as stainless steel or composite materials can becoated and similar reductions in bone cement bond strength to theprosthesis can be expected.

The femoral hip joint prosthesis of the present invention is coatedwith, for example, diamond-like carbon and implanted in the femur bone.As is customary, the femur bone is prepared by reaming a canal intowhich PMMA such as Simplex P® from Howmedica Inc., or other suitablebone cement, is introduced under pressure. Promptly after introductionof the bone cement into the canal and before the cement has had anopportunity to set, the stem of the femoral hip joint prosthesis isinserted into the cement with the result that a cement mantle is formedaround the stem up to the arcuate area and a portion of the enlargedshoulder. Any excess cement is wiped away leaving an exposed upper end.The free or distal end of the stem may be engaged in a hollow plasticcentralizer which insures that there will be a sufficient thickness ofcement around all portions of the stem or a preformed sheath as shown inU.S. Pat. No. 5,197,990. The optional plastic centralizer includes acup-shaped pocket having a plurality, preferably 3 or 4, of integrallyformed resilient arms sized to engage the interior of the canal. Thehollow cup-shaped pocket of the centralizer may be filled with acompressible material such as Avitin Powder, Surgicell, Gelfoam or thelike such that there will be no interference with subsidence of thedistal end of the prosthesis within the hollow pocket of thecentralizer. Prior to introduction of cement in the canal, a cementrestrictor may be positioned therein.

On the implanted femoral hip joint prosthesis after an extended period,say ten years, following implantation, there usually occurs a smallamount of radiological subsidence, on the average of 2 mm, where thestem has subsided within the cement mantle. Such subsidence within thecement mantle results in the distal end moving further into thecentralizer and in the enlarged shoulder pulling away from the cementmantle leaving a gap. Because of the tapered stem, collarless design ofCo-Cr-Mo alloy having a highly polished surface coated with diamond-likecarbon or the other coatings set forth above, the femoral hip jointprosthesis of the present invention is permitted to more easily subsidewithin the cement mantle end to do without disrupting the cement-boneinterface. Thus, the subsidence of the stem results in microscopicmovement of the stem in relation to the adjacent surface of the cementmantle. As will be appreciated, the effect of such microscopic movementis to cause the stem to self-tighten as it and the cement mantle subsideand to impart primarily compressive forces against the cement mantle indirections substantially normal to the interior surfaces of the bone.

While several examples of the present invention have been described, itis obvious that many changes and modifications may be made thereunto,without departing from the spirit and scope of the invention.

We claim:
 1. A femoral hip joint prosthesis having reduced adhesion tobone cement comprising:a proximal stem portion; and a distal stemportion for contact exclusively with bone cement lining a bone cavity,said distal stem being formed from metal having a polished surfacefinish coated with a discrete layer of diamond-like carbon to reduceadhesion to bone cement, said layer of diamond-like carbon being fromabout 1 to 30 microns thick with an outer surface maintaining saidpolished surface finish.
 2. The femoral hip joint prosthesis of claim 1wherein said layer of material is greater than 1 micron thick.
 3. Thefemoral hip joint prosthesis of claim 2 wherein said layer of materialis from about 1 to 3 microns thick.
 4. A femoral hip prosthesis havingreduced adhesion to bone cement comprising:a head and neck proximalportion; and an elongated stem portion for contact exclusively with bonecement lining a bone cavity extending from said proximal portion to adistal end, said stem having a medial, lateral, anterior and posteriorsurfaces, said surfaces tapering downwardly from said proximal portionto said distal end, said stem being formed from metal having a polishedsurface finish coated with a discrete layer of diamond-like carbon toreduce adhesion to bone cement, said layer of material is from about 1to 3 microns thick with an outer surface maintaining said polishedsurface finish.
 5. The femoral hip joint prosthesis of claim 4 whereinthe polished surface has a roughness of less than about 4 microinches.6. The femoral hip joint prosthesis of claim 5 wherein said layer ofmaterial is greater than 100Å thick.
 7. The femoral hip joint prosthesisof claim 6 wherein said layer of material is from about 1 to 3 micronsthick with an outer surface maintaining said polished surface roughness.